As is commonly known, a polyimide film has various excellent properties such as heat-resistance, cold resistance, chemical-resistance, insulation and mechanical strength. Therefore, a polyimide film is widely used as electronic materials such as an electric insulating film, a flexible printed wiring board and the like.
To produce an electric insulating film and a flexible printed wiring board, a polyimide film can be bonded to a copper foil by using acrylic or epoxy-base flexible adhesive agent, or it can be bonded directly to a metal layer without using any adhesive agents by vacuum evaporation or spatter evaporation. In any cases to produce a flexible printed wiring board or the like, it is inevitable that a polyimide film be bonded well together to an adhesive layer and/or a metal layer.
As a flexible printed wiring board produced by using a polyimide film is often exposed to an atmosphere of high temperature and high humidity, there has been a strong demand for a flexible printed wiring board having a strong adhesive property, which is not spoiled even under high temperature and high humidity.
Various attempts to improve an adhesive strength between a polyimide film and an adhesive agent or a metal layer has been made. One of the attempts is that the surface of a polyimide film is roughened and another attempt is that reactive functional group is introduced to the surface of a polyimide film by applying such techniques as corona discharge treatment, plasma treatment, or chemical surface treatment. However, in any attempts, it is difficult to keep an adhesive strength from deteriorating under an atmosphere of high temperature and high humidity.
In order to prevent an adhesive property from deteriorating under high temperature and high humidity, various inorganic or organic metallic compounds have been added to a polyimide film to control a higher-order structure of polyimide.
For example, U.S. Pat. No. 4,742,099 discloses a polyimide film containing titanic organo-compound to improve an adhesive strength of a polyinide film.
JP Pat. No. 7-299883 discloses a polyimide film produced by adding a organic tin compound to a polyamide acid solution or a polyamide acid polymer not to spoil a high bonding strength and a high adhesive strength of a polyimide film after aging under an atmosphere of high temperature and high humidity.
JP Pat. No. 7-503984 also discloses a polyimide film produced by adding an organometallic compound such as tin compound and the like with polyamide acid polymer before a casting step, to improve an adhesive strength of a polyimide film after aging under an atmosphere of high temperature and high humidity.
Furthermore, in order to provide a polyimide film with other properties than an adhesive property, various metallic compounds are often added.
U.S. Pat. No. 3,073,784 to A. L. Endrey et. al discloses that a polyimide film with an improved electric conductivity, which is produced by adding silver salt to an organic solvent solution of polyamide acid, curing the obtained solution and simultaneously reducing the size of a metallic particle to 0.8 .mu.m to keep a polyimide film transparent.
Macromolecules, 17, pp. 1627-1632(1984) discloses a polyimide film containing stannic chloride or dibutyltin chloride to decrease an electric resistance of the film.
Larry T. Taylor et. al (K. L. Mittal POLYIMIDES Synthesis, Characterization, and Applications) report that a rise of Tg and the improvement of mechanical strength under a high temperature can be seen by adding Al(acac).sub.3, AgNO.sub.3, Li.sub.2 PdCl.sub.4 and AuI.sub.3 to polyimide.
Robin E. Southward et. al report that adding fusible lanthanoid (III) acetate trihydrate to a polyimide film makes the coefficient of linear expansion of a polyimide film small.
Furthermore, polyimide molding has also various properties such as abrasion resistance, chemical resistance, radioactive resistance in addition to excellent heat resistance. Therefore, it is widely used as mechanism elements such as slide members, or automobile parts, office appliance, electrical and electric equipment, apparatus of aerospace sector and atomic sector, and general industrial machinery. However, to further use a polyimide molding as an alternate material of metal, precision mechanism element and the like, it is inevitable to improve mechanical properties of a polyimide molding such as mechanical strength and toughness and it is important to add or improve these properties by adding various metallic compounds.
As described above, some attempts to add various metallic compound have been made to improve properties of a polyimide film and a polyimide molding and to provide them with a novel property.
Generally, polyimide is prepared from polyimide acid, a precursor of polyimide, by dehydration. A polyimide film is obtained through the following steps:
mixing an organic solvent solution of polyamide acid with a dehydrating and cyclizing agent and casting a mixture to form a film; and heating a film-shape mixture at a high temperature after drying it. There are 4 types of methods to add various inorganic or organic metallic compounds in these steps as follows. PA1 R.sub.11 : --H or C.sub.3.about.8 hydrocarbon residue PA1 R.sub.12, R.sub.13 : C.sub.3.about.18 hydrocarbon residue PA1 R.sub.14 : C.sub.3.about.18 hydrocarbon residue, ##STR6## PA1 R.sub.15, R.sub.16 : C.sub.3.about.18 hydrocarbon residue PA1 R.sub.17 : C.sub.2.about.18 hydrocarbon residue PA1 1. Inhibitor for gelation is added and blended in the course of synthesizing polyamide acid, and inorganic or organic metallic compounds are added after polyamide acid is completely synthesized. PA1 2. Inhibitors for gelation are added and blended after polyamide acid is completely synthesized, and inorganic or organic metallic compounds are blended with the inhibitors for gelation or added and blended after adding the inhibitors for gelation. PA1 1. Inhibitors for gelation are added and blended in the course of synthesizing polyamide acid, and inorganic or organic metallic compounds are added to an organic solvent solution of polyamide acid alone or blended with curing agents. PA1 2. Inhibitors for gelation are added to and blended in the solution in which polyamide acid is completely synthesized, and inorganic or organic metallic compounds are added to said solution alone or they are added to the solution after being mixed in curing agents(dehydrating and cyclizing agents and catalysts). PA1 3. After polyamide acid is synthesized, inhibitors for gelation are added to and blended in curing agents (dehydrating and cyclizing agents and catalysts), and then the blend is added to an organic solvent solution of polyamide acid, and inorganic or organic metallic compounds are added to the solution alone or blended in the curing agents.
One of the methods is to add various inorganic and organic metallic compounds to an organic solvent solution of polyamide acid, a precursor of polyimide.
Another method is to add various inorganic and organic metallic compounds at the early stage of synthesizing polyamide acid, a precursor of polyimide.
Another method is to add various inorganic and organic metallic compounds to a dehydrating and cyclizing agent and catalyst.
The other method is to coat a partially-cast or partially-dried base film of polyamide acid with an organic solvent solution of metal salt, and then to heat and dry the coated base film while completely conversing said polyamide acid into polyimide.
However, in any above methods except the last one, when inorganic or organic metallic compounds are added to an organic solvent solution of polyimide acid, cross-link between metallic atom and functional group of polyamide acid is formed. As the cross-link is formed between macro-moleculars, the gelation of organic solvent solution of polyamide acid takes place, which makes it difficult to cast the organic solvent solution of polyamide acid to form a film.
The organic solvent solution of polyamide acid may possibly be cast, however, a gelling part of organic solvent solution of polyamide acid leads to problems in appearance (e.g. a black spot and uneven thickness, etc.) in the process of heating the film.
In addition to the problems in appearance, such properties as electric insulation, chemical-resistance, mechanical strength and the like degrade considerably.
In the case of the last method, there is no problems to cast an organic solvent solution of polyamide acid to form a film. However, when a partially-cast or partially-dried base film of polyamide acid is coated with an organic solvent solution of metal salt, partial gelation takes place on the surface layer of the film. A gelling part leads to problems in appearance and furthermore such properties as electric insulation, chemical-resistance, mechanical strength and the like degrade considerably.